Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen I), “Large-Scale Synthesis Of Carbon Nanotubes,” Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), “Carbon Nanotubes,” Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having up to seven walls have been described in the prior art. Ebbesen II; Iijima et al., “Helical Microtubules Of Graphitic Carbon,” Nature, Vol. 354, p. 56 (Nov. 7, 1991).
In defining single-wall carbon nanotubes, it is helpful to use a recognized system of nomenclature. In this application, the carbon nanotube nomenclature described by M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Chap. 19, especially pp. 756–760, (1996), published by Academic Press, 525 B Street, Suite 1900, San Diego, Calif. 92101-4495 or 6277 Sea Harbor Drive, Orlando, Fla. 32877 (ISBN 0-12-221820-5), which is hereby incorporated by reference, will be used. The single wall tubular fullerenes are distinguished from each other by double index (n,m) where n and m are integers that describe how to cut a single strip of hexagonal “chicken-wire” graphite so that its edges join seamlessly when it is wrapped to form a cylinder. When the two indices are the same, m=n, the resultant tube is said to be of the “arm-chair” (or n,n) type, since when the tube is cut perpendicular to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. Arm-chair tubes are one form of single-wall carbon nanotubes; they are truly metallic, and have extremely high electrical conductivity. Other nanotube geometries, where (n−m)/3 is an integer are semi-metallic, i.e. they have a small band-gap and are good electrical conductors at temperatures relevant to the operation of almost all electronic materials and devices. The remaining nanotube geometries where (n−m)/3 is not an integer are semiconductors, having a band-gap in the neighborhood of 1 eV, which varies with inversely with their individual diameters. See Odom et al, J. Phys. Chem. B, vol. 104 p. 2794 (2000). In addition, all single-wall nanotubes are the stiffest molecules known, and have extremely high thermal conductivity and tensile strength. See Yakobson and Smalley, Am. Sci. vol. 85, p. 324 (1997).
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., “Single-Shell Carbon Nanotubes of 1 nm Diameter,” Nature, Vol. 363, p. 603 (1993); Bethune et al., “Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,” Nature, Vol. 63, p. 605 (1993); Ajayan et al., “Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis,” Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., “Single-Walled Carbon Nanotubes Growing Radially From YC2 Particles,” Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al., “Single-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters,” Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., “Carbon Nanocapsules Encaging Metals and Carbides,” J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., “Extrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source,” Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., “Improving Conditions Toward Isolating Single-Shell Carbon Nanotubes,” Chem. Phys. Lett., Vol. 226, p. 364 (1994).
While this arc discharge process can produce single-wall nanotubes, the yield of nanotubes is low and the tubes exhibit significant variations in structure and size between individual tubes in the mixture. Individual carbon nanotubes are difficult to separate from the other reaction products and purify.
An improved method of producing single-wall nanotubes is described in U.S. Pat. No. 6,183,714, entitled “Method of Making Ropes of Single-Wall Carbon Nanotubes,” incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce single-wall carbon nanotubes in yields of at least 50% of the condensed carbon. The single-wall nanotubes produced by this method are much more pure than those produced by the arc-discharge method. Because of the absence of impurities in the product, the aggregation of the nanotubes is not inhibited by the presence of impurities and the nanotubes produced tend to be found in clusters, termed “ropes,” of 10 to 5000 individual single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular lattice.
PCT/US/98/04513 entitled “Carbon Fibers Formed From Single-Wall Carbon Nanotubes” and which is incorporated by reference, in its entirety, discloses, inter alia, methods for cutting and separating nanotube ropes and manipulating them chemically by derivatization to form devices and articles of manufacture comprising nanotubes. Other methods of chemical derivatization of the side-walls of the carbon nanotubes are disclosed in PCT/US99/21366 entitled “Chemical Derivatization of Single Wall Carbon Nanotubes to Facilitate Solvation Thereof, and Use of Derivatized Nanotubes,” which is incorporated by reference, in its entirety.
Another method for producing single-wall carbon nanotubes is described in PCT/US99/25702, (entitled “Gas-Phase Nucleation and Growth of Single-Wall Carbon Nanotubes from High Pressure CO”) incorporated herein in its entirety by reference, which describes a process involving supplying high pressure (e.g., 30 atmospheres) CO that has been preheated (e.g., to about 1000° C.) and a catalyst precursor gas (e.g., Fe(CO)5) in CO that is kept below the catalyst precursor decomposition temperature to a mixing zone. In this mixing zone, the catalyst precursor is rapidly heated to a temperature that results in (1) precursor decomposition, (2) formation of active catalyst metal atom clusters of the appropriate size, and (3) favorable growth of single-wall carbon nanotubes (“SWNTs”) on the catalyst clusters. Preferably a catalyst cluster nucleation agency is employed to enable rapid reaction of the catalyst precursor gas to form many small, active catalyst particles instead of a few large, inactive ones. Such nucleation agencies can include auxiliary metal precursors that cluster more rapidly than the primary catalyst, or through provision of additional energy inputs (e.g., from a pulsed or CW laser) directed precisely at the region where cluster formation is desired. Under these conditions SWNTs nucleate and grow according to the Boudouard reaction. The SWNTs thus formed may be recovered directly or passed through a growth and annealing zone maintained at an elevated temperature (e.g., 1000° C.) in which tubes may continue to grow and coalesce into ropes.
In yet another method for production, single-walled carbon nanotubes have been synthesized by the catalytic decomposition of both carbon monoxide and hydrocarbons over a supported metal catalyst. Under certain conditions, there is no termination of nanotube growth, and production appears to be limited only by the diffusion of reactant gas through the product nanotube mat that covers the catalyst. “Catalytic Growth of Single-Wall Carbon Nanotubes from Metal Particles” (PCT/US99/21367) incorporated in its entirety by reference, details a catalyst-substrate system that promotes the growth of nanotubes that are predominantly single-walled tubes in a specific size range, rather than the large irregular-sized multi-walled carbon fibrils that are known to grow from supported catalysts. This method allows bulk catalytic production of predominantly single-wall carbon nanotubes from metal catalysts located on a catalyst-supporting surface.
Carbon nanotubes, and in particular, single-wall carbon nanotubes, are useful for making electrical connectors in micro devices such as integrated circuits or in semiconductor chips used in computers because of the electrical conductivity and small size of the carbon nanotube. The carbon nanotubes are useful as antennas at optical frequencies, as constituents of non-linear optical devices, and as probes for scanning probe microscopy such as are used in scanning tunneling microscopes (STM) and atomic force microscopes (AFM). The carbon nanotubes may be used in place of or in conjunction with carbon black in tires for motor vehicles, as elements of a composite materials to elicit specific physical, chemical or mechanical properties in those materials (e.g. electrical and/or thermal conductivity, chemical inertness, mechanical toughness, etc). The carbon nanotubes themselves and materials and structures comprising carbon nanotubes are also useful as supports for catalysts used in industrial and chemical devices and processes such as fuel cells, hydrogenation, reforming and cracking.
Individual SWNT and ropes of single-wall carbon nanotubes exhibit metallic conductivity, i.e., they will conduct electrical charges with a relatively low resistance. Nanotubes and ropes of nanotubes are useful in any application where an electrical conductor is needed, for example as an additive in electrically conductive polymeric materials, paints or in coatings. Nanotubes and ropes of nanotubes have a propensity to aggregate into large networks, which are themselves electrically conductive, and this property enables them to form such networks when they are suspended in a matrix of a different material. The presence of this network alters the electrical properties of a composite that includes nanotubes.
Single-wall carbon nanotubes have outstanding properties as field-emitters for electrons, and serve well as the active element in cold-cathodes in any applications that involve emission of electrons, such as microwave power tubes and video displays.